Morgan Spurlock says he’ll figure out just what causes all those female fans to lose their minds when they see the boys of One Direction in his new 3-D doc on the boy band, “This is Us.” “We ask the question ‘Why them?’ ” the “Supersize Me” director told us Friday at the Tribeca Film Festival’s Disruptive Innovation Awards at NYU Stern’s Paulson Auditorium. “We talked to this one doctor who tells us what happens inside a girl’s body when she hears One Direction — like what happens in her brain and what goes on inside of her.” The movie, which also promises shirtless footage of the band’s leading heartthrob, Harry Styles, (we can hear the shrieking now), is out Aug. 30.